A controller for detecting the end of an operation (often called the "endpoint") on materials and particularly on semiconductor wafers will typically detect the endpoint by detecting a change in light being reflected or transmitted from the material. In one system for doing this an optical emitter such as a light-emitting diode (LED) produces light which strikes a wafer surface and is reflected back to a photodetector. In another system light produced by a reaction process is monitored by a detector and the rate of change of this light in certain frequency bands is used to detect the endpoint of an operation. In both cases the detected light intensity is a measure of the state of the material being processed. The state of the material being processed may be measured by the material's reflectivity or by the chemical constituents of the material or by the index of refraction of the material. A change in reflectivity indicates the process endpoint for metal etching, while the end of thin-film-interference oscillation in the detected light may signal the endpoint for dielectric etching and photoresist development.
During development and removal of photoresist or during dielectric etching, interference fringes are a direct indication of resist dissolution or dielectric removal. The breakthrough to the underlying substrate (which might be a semiconductor wafer surface, for example) that occurs when the photoresist or dielectric is removed is referred to as the endpoint and, as shown in FIG. 1, is recognizable as the point where the interference signal becomes nearly flat. The total process time consists of the time (A) which is required to reach breakthrough (endpoint), and any additional time (B) needed to clear out the resist or the dielectric completely. The time (B) is generally referred to as the overdevelopment period and will depend upon the nature of the material being removed. The total process time is equal to the sum of the time to endpoint plus the overdevelopment or overetch time. For simplicity, the phrase "overprocess time" will be used in this specification to mean either the overdevelopment time or the overetch time depending on whether a photoresist is being developed and thus removed or a layer of material is being removed.
The actual signals observed can, and in many cases will, vary drastically from an ideal interferogram pattern. Variations in reflectivity from the substrate layers (Si, Poly-Si, Al, SiO.sub.2, Si.sub.3 N.sub.4, etc.) die density, topography, substrate roughness, as well as process variables, will affect the strength and characteristics of the reflected signal.
Establishing precise endpoint time is essential in determining the start of the overprocess period, so that the total process time is tightly controlled. A reliable and accurate process control system must be able to recognize an endpoint under any variable signal conditions. For instance, in semiconductor processing where endpoint detection is important, wafer conditions and the wafer-to-sensor distance may vary. In order for the endpoint to be accurately detected, small changes in the reflected light intensity must be measured and changes in light intensity due to effects other than the process being monitored must be eliminated.
Endpoint detection sensors capable of meeting these stringent requirements can suffer from several problems. Ambient light can interfere with the detection of light reflected from the wafer. In addition, as the wafer spins during processing, the wafer may wobble or tilt, causing the reflected light intensity to vary. This artifact may also interfere with endpoint detection.
Sensor boards for processing the electrical signals produced by the photodetectors of endpoint detection sensors according to the prior art may only have a dynamic range of 4. In this case, strong signals caused by intense light reflected from the substrate may saturate the detection circuitry of the sensor board and prevent accurate recording of light intensities. Alternatively, weak signals and small signal changes may be too small for accurate measurement or detection if those weak signals cannot be adequately amplified. In addition, it may not be possible to adjust the sensor distance from the substrate surface to a convenient level to avoid, for instance, the developer dispense nozzle when only a limited dynamic range is available. A sensor placed far from a substrate will receive less intense light and unless the signal generated by that light can be adequately amplified, the signal may be too weak to provide accurate measurement.